Variable pulley transmissions for transferring torque from an input shaft to an output shaft have been used for some time. In such arrangements a first pulley is mounted on the input shaft, and this pulley has at least one flange axially movable relative to its other flange to change the effective pulley diameter. A second, similarly adjustable pulley is mounted on the output shaft. A flexible belt intercouples the two pulleys to transfer torque between them. As the effective diameter of one pulley is changed, and simultaneously the effective diameter of the other pulley is changed in the opposite direction, the drive ratio between the input and output shafts is adjusted in a smooth, continuous manner.
For over 30 years automotive engineers have recognized that the maximum operating efficiency of the engine could be achieved if the transmission could be controlled by adjusting to different loads and speed ratios such that the engine is maintained at its maximum efficiency point. This is not possible with a conventional geared transmission in which the drive ratio is adjusted in discrete steps, rather than continuously. Accordingly, efforts have been directed to the use of a continuously variable transmission of the type described above. This has resulted in the production and marketing in Europe of the Daf passenger car, using a flexible rubber belt in such a continuously variable transmission (CVT). Such a belt must be relatively wide because of the torque it must handle, and operates under severe temperature, vibration and other adverse conditions. Accordingly, efforts have been channeled to produce a flexible belt of metal, and some of these efforts are described in the patent literature. Such belts can be relatively narrow and possess more load-torque handling capacity than a rubber belt of the same cross-section and, in addition, provide greater efficiency in transmitting torque between the pulleys of the transmission.
A chain-belt for a pulley transmission comprising ranks or sets of links interleaved with other sets of links and connected by pivot means, and drive or load blocks of generally trapezoidal shape located between adjacent pivot pins to transmit load to the pins, is disclosed and claimed in U.S. Pat. No. 4,313,730, issued Feb. 2, 1982 to Cole et al. In this patent, the drive or load blocks have a centrally located "window" through which links of the chain pass, and angled side edges which drivingly engage the pulley flanges. This belt is of the "pull" or tension type because the drive or load blocks do not move longitudinally along the chain but are constrained by the pivot means.
Another type of metal drive belt for a pulley transmission is taught in U.S. Pat. No. 3,720,113. This belt comprises a flexible band of superimposed, nested steel strips with metal blocks longitudinally movable thereon. Each of the blocks has tapered edges to engage the flanges of the pulleys of the transmission. Because the metal blocks abut each other and are under compression when torque is transferred, the belt is classified as a "push" or compression type. The projected cost of the described "push" type belts is several times the cost of a "pull" chain-belt as taught by Cole et al. Thus, economically, the Cole et al chain-belt is much more attractive than the belt of U.S. Pat. No. 3,720,113.
One major factor to be considered in using the CVT for automotive use is the noise generated by the engagement of drive blocks and pulley flanges. Noise which "peaks" at certain frequencies in a regular pattern can be disagreeable and annoying to humans. Acoustic research results have proven that a white, irregular or arrythmic noise is less noticeable and annoying than a noise which contains recognizable pure tones or a single frequency signal. The most annoying chain noise is the single tone or frequency which may occur above 100 Hz, and has an amplitude extending above the average amplitude by 5 to 10 dB. Some of the prior art chain-belts described may produce a noise spectrum including signals which are annoying to humans.